Because the receiving capability of a plurality of antennas which construct an array antenna degrades remarkably when amplitude errors and phase errors occur among signals received by the plurality of antennas, calibration devices that compensate for such amplitude errors and phase errors have been developed.
A calibration device disclosed in the non-patent literature 1 as listed below uses a calibration method of estimating an amplitude error and a phase error occurring in each antenna by injecting a signal for calibration from between each antenna and a receiving unit, extracting a signal for calibration outputted from each receiving unit, and making a comparison between those signals for calibration.
However, this calibration method cannot eliminate an error which each antenna body has. Further, because an amplitude error and a phase error occurring in each antenna have secular changes, it is necessary to provide a mechanism for injecting a signal for calibration and making a comparison between signals for calibration at regular intervals.
As a calibration method of compensating for amplitude errors and phase errors occurring in the whole of a device including antenna bodies and receiving units, there is a method of receiving a signal whose direction of arrival is known and which is emitted from a radiation source by using a plurality of antennas, and making a comparison in amplitude and in phase among signals received by the plurality of antennas, thereby estimating an amplitude error and a phase error occurring in each of the antennas.
However, in this calibration method, it is necessary to prepare a plurality of radiation sources that emit signals whose directions of arrival are known. Further, in order to deal with secular changes in the amplitude error and the phase error occurring in each of the antennas, it is necessary to repeatedly carry out the above-mentioned estimating process at regular intervals.
In the non-patent literature 2 as listed below, as a process for solving the above-mentioned problem, a method of performing a calibration by using a radiation source that emits a signal whose direction of arrival is unknown is disclosed.
According to this method, on the assumption that the amplitude error and the phase error occurring in each of the antennas do not have an angular dependence, a model in which received signals are multiplied by an array error matrix expressed by the dimensions which are (the number of array elements)×(the number of array elements) is generated, and a calibration is performed by estimating the array error matrix.
However, it is described in the non-patent literature 1 that in this method, the assumption is made that the array error matrix is a diagonal matrix, but this assumption is not established under a complicated reflection environment in the vicinity of the sensor.